The term “film capacitors” relates to a family of capacitors which are wound, wrapped or molded with films comprising of metals deposited or laminated on thin insulating materials, usually polymer films such as polypropylene. It is well known that the dielectric strength of many insulating films used in film capacitors rises as the temperature is reduced. FIG. 1 shows data for various films, although the group of films represented in these figures is not exhaustive. Many other film materials with similar temperature behavior are also used in capacitors.
What is not well documented is the fact that a capacitor wrapped with these films exhibits improved performance at cryogenic temperatures. First and foremost, the voltage capability increases as the device is cooled. This leads to much higher energy storage in cryogenically operated capacitors, since the stored energy is proportional to the square of the operating voltage:E=½CV2.
The inventors have operated conventional, off-the-shelf film capacitors (manufactured and rated for use around room temperature) at more than twice their rated voltages. The energy storage capability in these devices therefore increased by a factor of four compared to room-temperature operation. This phenomenon is demonstrated in FIG. 2, which shows a dramatic reduction in the leakage current of a cryogenically operated capacitor. Increasing leakage current with increasing voltage is a direct indicator that the device is nearing its breakdown limit.
The equivalent series resistance (ESR) of these capacitors is also reduced at low temperatures as shown by our measurements in FIG. 3. The results are normalized with respect to 300-K operation. This improvement may be a result of the increased conductivity in the plate and terminal materials or reduction of dielectric losses or both. Capacitor plate and terminal materials with conductivities that are enhanced by low temperatures (including superconductors) will further improve the ESR. Dielectric materials whose losses decrease at low temperatures can also experience a resultant improvement in ESR. FIG. 4 gives the loss tangents of various films at several temperatures, and illustrates the improvement at cryogenic temperatures, especially at 77 K (liquid nitrogen) and at 4.2 K (liquid helium). Note the improvement of the loss factor for polypropylene (PP), which decreases by a factor of 30 when cooled from room temperature to 77 K. Most film capacitors are made with PP.
It is important to note that the low-temperature behavior of an intrinsic dielectric material does not always translate into the same behavior for a manufactured device. The metallization process, for example, can greatly degrade the dielectric strength of polymer films, and thus the final capacitor can have a much lower breakdown voltage than one would expect. Cryogenic operation of capacitors can help offset this deterioration.